Some things you can edit online and some you can't under RsLogix 5000.
First off, load the program and go online as you normally do, leaving the processor in remote/run mode. You have to have the keyswitch on the processor in remote/run. If you don't know how to do this, you turn the key to RUN then back to REM and leave it there. When you get online your status indicator will show Rem Run - the default location is the upper left corner of the screen.
Now that you are online with the processor in the proper mode, you can add and delete rungs or parts of rungs as you see fit. You can also add tags and alias them to existing physical I/O or other tags. You can edit tags and change the name or description but once created you can't change the alias. Also there are some wierd rules about some tags you can't change at all that I am not going to go into here.
A few things to consider:
When adding or deleting rungs, please be sure that what you are changing is not going to cause unexpected operation of equipment that could get someone hurt. Adding a rung in this mode generally causes the processor to immediately execute the rung. Also, the RSLogix program will generally catch incomplete rungs and prevent you from compiling them but it does not check your logic - after all, the program has no idea what you are trying to do. Also the program will not catch an indirect addressing fault until it tries to step through the rung but by then it's too late and will fault the processor. (At least that's what it did on PLC 5 and SLC 500...I'm not so sure about RSLogix 5000 as I avoid indirect addressing anymore.)
Whew! Sorry about being long-winded but that's the basic concept. I'm sure others on here will have some to add to what I said or correct me on things I got wrong.